It is important for clinical chemical texts in the present medical treatment to quantitatively determine various analytes in body fluids (whole blood, plasma, serum, urine, seliva, etc.) so as to make accurate diagnosis and proper treatment. For the purpose of performing such quantitative analysis, biochemical analytical methods using a multilayer analytical element in the form of a slide containing an analytical element or a continuous analytical element film and spectroscopic measurement have been proposed and put to practical use.
In a quantitative analysis using a multilayer analytical element, a small amount of a liquid sample to be analyzed is applied (spotted) on a multilayer analytical element and incubated at a given temperature for a given period of time to cause a color reaction between an analyte in the liquid and a reagent in the element. Then the analyte is quantitatively determined by spectroscopically measuring a reflection optical density of the color formed by the reaction.
As a light source for measuring reflection density, there is advantageously used a source which emits a pulsed light, such as a pulse xenon source, which has the advantage of generating a light usable for the analysis having a high intensity in proportion input to power and evolved heat. The pulse xenon source is particularly favorable for giving a light at a wavelength of 300-400 nm, which is important for certain analytical elements. Reflectance in the measurement of reflection density is determined as a ratio of an intensity of light reflected from a surface of an article (object) to be measured to an intensity of light generated by a light source (namely, an intensity of reference light). The intensity of a reference light is heretofore determined by irradiating separately a white board with a light generated by the same light source and measuring an intensity of light reflected therefrom. The reflection density is not always determined accurately by measuring the intensity of reference light in that way, because the intensity of light generated by the source of pulsed light varies, in a strict sense, for every pulse and the measured intensity of light does not always precisely coincide with the intensity of light which is used for the irradiation of the article. Alternatively, the reference light can be taken out of a light of the light source using an optical fiber or the like. However, thus obtained reference light is not reliably employable, because the intensity of the generated light is not always uniformly distributed around a site from which the light is generated.
Accordingly, it is desired to precisely measure the intensity of reference light every time the reflection density is determined for obtaining exact results of the analysis.